As is known, the formation of extruded foam cellular products such as foam boards, planks and billets is enhanced by the employment of a vacuum chamber so that expansion of foaming material exiting an extrusion die takes place under sub-atmospheric pressure. To this end and because of the delicate and fragile nature of the extruded foam, vacuum extrusion lines have employed an inclined barometric leg of substantial length which, at its upper end, defines a vacuum chamber into which the foaming material is extruded and formed to its final shape before it is guided by a conveyor along the length of the barometric leg. At its lower end, the barometric leg extends at a shallow angle into a pool of water, and the conveyor continues through a large radius in the pool to guide the foam extrudate through and from the pool for further processing such as cutting to size and length. In a full size foam extrusion line, the barometric leg may be more than 50 meters in length and the large radius portion of the conveyor in the pool may be more than 30 meters in length. Accordingly, the foam extrudate may have a length of more than 80 meters by the time the lead end thereof exits the pool.
Because the extruded foam generally assumes a cross-section different than that of the die orifice through which it is extruded, extrusion dies advantageously have employed a pair of adjustable, rigid die lips which together define therebetween an elongated, axially outwardly extending arcuate die orifice through which the foaming material moves both internally and axially outwardly relative to the die lips. Since the die lips necessarily are located inside the vacuum chamber and thus inaccessible during extrusion, provision has been made for remotely adjusting the die lips from outside the vacuum chamber. The purpose of adjusting is to vary the size and shape of the die orifice which controls to a substantial degree the thickness and profile of the extruded product.
As is a customary practice, the die lips initially are set at the beginning of a production run to provide a die orifice size and shape which past experience has found will approximately give the desired thickness and profile of the extruded product. The size and shape of the die orifice however will usually change because of heat and pressure expansion and any looseness or backlash freedom in the die lip adjusting mechanisms when the die lips and associated hardware are subjected to the high temperature and pressures of the extrusion process. This accordingly necessitates adjustment of the die lips.
Since the die lips inside the vacuum chamber and inaccessible to the extrusion line operator, adjustments made from outside the vacuum chamber are indeterminate as no die orifice measurements can be made. This necessitates the making of several die adjustments, each constituting an educated guess as to what the adjustment will produce in terms of die orifice size and shape and resultant product thickness and profile. Accordingly, measurements of the final product thickness and profile must be taken and corrective die lip adjustments made to bring the product within specification. As was customary practice, product measurements were taken after the product could be gotten to upon exiting the pool at the end of the barometric leg. As a result, a substantial length of off-specification or scrap product, such as more than 80 meters as indicated above, would have to be run before measurements were taken and the extrusion die adjusted accordingly. Moreover, the results of any adjustment would not be determined until an additional length of product was run and measurements taken some five to 20 minutes later. Such procedure, needless to say, results in substantial scrap and lost time.